So 得 (děi, have to) and 必须 (bìxū,must) share the same mean. “必须 “is more formal than”得“. “得”Somewhat similar to 必须(bìxū, must) but much more colloquial. Actually “得” is a dialect in north China, it is often used in daily life and is not used in formal writing.

The negative form of必须 and 得 is不用(bú yòng), which would be equivalent to English don’t have to.

Yes, you can use 不必(bú bì, not necessary) too.不用=不必

Eg. 1、我明天不用早起（Wǒ míngtiān bú yòng zǎoqǐ, I don’t have to get up early tomorrow.）